Network Working Group J. Klensin
Request for Comments: 4290 December 2005
Category: Informational
Suggested Practices for Registration ofInternationalized Domain Names (IDN)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
IESG Note
This RFC is not a candidate for any level of Internet Standard. The
IETF disclaims any knowledge of the fitness of this RFC for any
purpose and notes that the decision to publish is not based on IETF
review apart from IESG review for conflict with IETF work. The RFC
Editor has chosen to publish this document at its discretion. See
RFC 3932 for more information.
Abstract
This document explores the issues in the registration of
internationalized domain names (IDNs). The basic IDN definition
allows a very large number of possible characters in domain names,
and this richness may lead to serious user confusion about similar-
looking names. To avoid this confusion, the IDN registration process
must impose rules that disallow some otherwise-valid name
combinations. This document suggests a set of mechanisms that
registries might use to define and implement such rules for a broad
range of languages, including adaptation of methods developed for
Chinese, Japanese, and Korean domain names.
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RFC 4290 IDN Registration Practices December 20051. Introduction1.1. Background
The IDNA (Internationalized Domain Names in Applications)
specification [RFC3490] defines the basic model for encoding non-
ASCII strings in the DNS. Additional specifications [RFC3491]
[RFC3492] define the mechanisms and tables needed to support IDNA.
As work on these specifications neared completion, it became apparent
that it would be desirable for registries to impose additional
restrictions on the names that could actually be registered (e.g.,
see [IESG-IDN] and [ICANN-IDN]) to reduce potential confusion among
characters that were similar in some way. This document explores
these IDN (international domain name) registration issues and
suggests a set of mechanisms that IDN registries might use.
Registration restrictions are part of a long tradition. For example,
while the original DNS specifications [RFC1035] permitted any string
of octets in a DNS label, they also recommended the use of a much
more restricted subset. This subset was derived from the much older
"hostname" rules [RFC952] and defined by the "LDH" convention (for
the three permitted types of characters: letters, digits, and the
hyphen). Enforcement of this restricted subset in registrations was
the responsibility of the registry or domain administrator. The
definition of the subset was embedded in the DNS protocol itself,
although some applications protocols, notably those concerned with
electronic mail, did impose and enforce similar rules.
If there are no constraints on registration in a zone, people can
register characters that increase the risk of misunderstandings,
cybersquatting, and other forms of confusion. A similar situation
existed even before the introduction of IDNA, as exemplified by
domain names such as example.com and examp1e.com (note that the
latter domain contains the digit "1" instead of the letter "l").
For non-ASCII names (so-called "internationalized domain names" or
"IDNs"), the problem is more complicated. In the earlier situation
that led to the LDH (hostname) rules, all protocols, hosts, and DNS
zones used ASCII exclusively in practice, so the LDH restriction
could reasonably be applied uniformly across the Internet. Support
for IDNs introduces a very large character repertoire, different
geographical and political locations, and languages that require
different collections of characters. The optimal registration
restrictions are no longer a global matter; they may be different in
different areas and, hence, in different DNS zones.
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For some human writing systems, there are characters and/or strings
that have equivalent or near-equivalent usages. If a name can be
registered with such a character or string, the registry might want
to automatically associate all of the names that have the same
meaning with the registered name. The registry might also decide
whether the names that are associated with, or generated by, one
registration should, as a group or individually, go into the zone or
should be blocked from registration by different parties.
To date, the best-developed system for handling registration
restrictions for IDNs is the JET Guidelines for Chinese, Japanese,
and Korean [RFC3743], the so-called "CJK" languages. The JET
Guidelines are limited to the CJK languages and, in particular, to
their common script base. Those languages are also the best-known
and most widely-used examples of writing systems constructed on
"ideographic" or "pictographic" principles. This document explores
the principles behind the JET guidelines. It then examines some of
the issues that might arise in adapting them to alphabetic languages,
i.e., to languages whose characters primarily represent sounds rather
than meanings.
This document describes five things:
1. The general background and considerations for non-ASCII scripts
in names.
2. Suggested practices for describing character variants.
3. A method for using a zone's character variants to determine which
names should be associated with a registration.
4. A format for publishing a zone's table of character variants;
Such tables are referred to below simply as "language tables" or
simply "tables".
5. A model algorithm for name registration given the presence of
language tables.
1.2. The Nature and Status of these Recommendations
The document makes recommendations for consideration by registries
and, where relevant, by those who coordinate them, and by those who
use their services. None of the recommendations are intended to be
normative. Instead, the intent of the document is to illustrate a
framework for developing variations to meet the needs of particular
registries and their processing of particular languages. Of course,
if registries make similar decisions and utilize similar tools, costs
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and confusion may be reduced -- both between registries and for users
and registrars who have relationships with more than one domain.
Just as the JET Guidelines contain some suggestions that may not be
applicable to alphabetic scripts, some of the suggestions here,
especially the more specific ones, may be applicable to some scripts
and not others.
1.3. Terminology1.3.1. Languages and Scripts
This document uses the term "language" in what may be, to many
readers, an odd way. Neither this specification, nor IDNA, nor the
DNS are directly concerned with natural language, but only with the
characters that make up a given label. In some respects, the term
"script", used in the character coding community for a collection of
characters, might be more appropriate. However, different subsets of
the same script may be used with different languages, and the same
language may be written using different characters (or even
completely different scripts) in different locations, so "script" is
not precisely correct either.
Long-standing confusion has also resulted from the fact that most
scripts are, informally at least, named after one of the languages
written in them. "Chinese" describes both a language and a
collection of characters that are also used in writing Japanese,
Korean, and, at least historically, some other languages. "Latin"
describes a language, the characters used to write that language,
and, often, characters used to write a number of contemporary
languages that are derived from or similar to those used to write the
Latin language. The script used to write the Arabic language is
called "Arabic", but it is also used (typically with some additions
or deletions) to write a number of other languages. Situations in
which a script has a clearly-defined name that is independent of the
name of a language are the exception, rather than the rule; examples
include Hangul, used to write Korean, Katakana and Hiragana, used to
write Japanese, and a few others. Some scholars have historically
used "Roman" or "Roman-derived" for the script in an attempt to
distinguish between a script and the Latin language.
The term "language" is therefore used in this document in the
informal sense of a written language and is defined, for this
purpose, by the characters used to write it, i.e., as a language-
specific subset of a script. In this context, a "language" is
defined by the combination of a code (see Section 1.4.1) and an
authority that has chosen to use that code and establish a
character-listing for it. Authorities are normally TLD (top-level
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domain) registries; see Section 7 and [IANA-language-registry].
However, it is expected that TLD registries will find appropriate
experts and that advice from language and script experts selected by
international neutral bodies will also become part of the
registration system. In addition, as discussed below in Section 7,
registries may conclude that the best interests of registrants,
stakeholders, and the Internet community would be served by
constructing "language tables" that mix scripts and characters in
ways that conform to no known language. Conventions should be
developed for such registrations that do not misleadingly reflect
specific language codes.
1.3.2. Characters, Variants, Registrations, and Other Issues
1. Characters in this document are specified by their Unicode
codepoints in U+xxxx format, by their official names, or both.
2. The following terms are used in this document.
* String
A "string" is an sequence of one or more characters.
* Base Character
This document discusses characters that may have equivalent or
near-equivalent characters or strings. A "base character" is
a character that has zero or more equivalents. In the JET
Guidelines, base characters are referred to as "valid
characters". In a table with variants, as described in
Section 5, the base characters occupy the first column.
Normally (and always, if the recommendation of Section 6.3 is
adopted), the base characters will be the characters that
appear in registration requests from registrants; any other
character will invalidate the registration attempt.
* Native Script
Native script is the form in which the relevant string would
normally be represented. For example, it might use Lower
Slobbovian characters and the glyphs normally used to write
them. It would not be punycode as a presentation form.
* Variant Characters/Strings
The "variant(s)" are character(s) and/or string(s) that are
treated as equivalent to the base character. Note that these
might not be exactly equivalent characters; a particular
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original character may be a base character with a mapping to a
particular variant character, but that variant character may
not have a mapping to the original base character. Indeed,
the variant character may not appear in the base character
list, and hence may not be valid for use in a registration.
Usually, characters or strings to be designated as variants
are considered either equivalent or sufficiently similar (by
some registry-specific definition) that confusion between them
and the base character might occur.
* Base Registration
The "base registration" is the single name that the registrant
requested from the registry. The JET Guidelines use the term
"label string" for this concept.
* Registered, Activated
A label (or "name") is described as "registered" if it is
actually entered into a domain (i.e., into a zone file) by the
registry, so that it can be accessed and resolved using
standard DNS tools. The JET Guidelines describe a
"registered" label as "activated". However, some domains use
a slightly different registration logic in which a name can be
registered with the registrar (if one is involved) and with
the registry, but not actually entered into the zone file
until an additional activation or delegation step occurs.
This document does not make that distinction, but is
compatible with it.
As specified in the IDNA Standard, the name actually placed in
the zone file is always the internal ("punycode") form. There
is no provision for actually entering any other form of an IDN
into the DNS. It remains controversial, with different
registrars and registries having adopted different policies,
as to whether the registration, as submitted by the
registrant, is in the form of:
o The native-script name, either in UTF-8 or in some coding
specified by the registrar, or
o the internal-form ("punycode") name, or
o both forms of the name together, so that the registrar and
registry can verify the intended translation.
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If any of the approaches defined in this document is used, it
is almost certain to be necessary that the native-script form
of the requested string be available to the registry.
* Registration Bundle
A "registration bundle" is the set of all labels that come
from expanding the base characters for a single name into
their variants. The presence of a label in a registration
bundle does not imply that it is registered. In the JET
Guidelines, a registration bundle is called an "IDN Package".
* Reserved Label
A "reserved label" is a label in a registration bundle that is
not actually registered.
* Registry"
A "registry" is the administrative authority for a DNS zone.
The registry is the body that enforces, and typically makes,
policies that are used in a particular zone in the DNS.
* Coded Character Set
A "Coded Character Set" (CCS) is a list of characters and the
code positions assigned to them. ASCII and Unicode are CCSs.
* Language
A "language" is something spoken by humans, independent of how
it is written or coded. ISO Standard 639 and IETF BCP 47 (RFC3066) [RFC3066] list and define codes for identifying
languages.
* Script
A "script" is a collection of characters (glyphs, independent
of coding) that are used together, typically to represent one
or more languages. Note that the script for one language may
heavily overlap the script for another. This does not imply
that they have identical scripts.
* Charset
"Charset" is an IETF-invented term to describe, more or less,
the combination of a script, a CCS that encodes that script,
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and rules for serializing encoded bytes that are stored on a
computer or transmitted over the network.
The last four of these definitions are redundant with, but
deliberately somewhat less precise than, the definitions in
[RFC3536], which also provides sources. The two sets of definitions
are intended to be consistent.
1.3.3. Confusion, Fraud, and Cybersquatting
The term "confusion" is used very generically in this document to
cover the entire range from accidental user misperception of the
relationship between characters with some characteristic in common
(typically appearance, sound, or meaning) to cybersquatting and
(other) deliberately fraudulent attempts to exploit those
relationships based on the nature of the characters.
1.4. A Review of the JET Guidelines1.4.1. JET Model
In the JET Guidelines model, a prospective registrant approaches the
registry for a zone (perhaps through an intermediate registrar) with
a candidate base registration -- a proposed name to be registered --
and a list of languages in which that name is to be interpreted. The
languages are defined according to the fairly high-resolution coding
of [RFC3066] or, if the registry considers it more appropriate, a
coding based on scripts such as those in [LTRU-Registry]. In this
way, Chinese as used on the mainland of the People's Republic of
China ("zh-cn") can, at registry option, consist of a somewhat
different list of characters (code points) and be represented by a
separate table compared to Chinese as used in Taiwan ("zh-tw").
The design of the JET Guidelines took one important constraint as a
basis: IDNA was treated as a firm standard. A procedure that
modified some portion of the IDNA functions, or was a variant on
them, was considered a violation of those standards and should not be
encouraged (or, probably, even permitted).
Each registry is expected to construct (or obtain) a table for each
language it considers relevant and appropriate. These tables list,
for the particular zone, the characters permitted for that language.
If a character does not appear as a base character (called a "valid
code point" in the JET document) in that table, then a name
containing it cannot be registered. If multiple languages are listed
for the registration, then the character must appear in the tables
for each of those languages.
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The tables may also contain columns that specify alternate or variant
forms of the valid character. If these variants appear, they are
used to synthesize labels that are alternatives to the original one.
These labels are all reserved and can be registered or "activated"
(placed into the DNS) only by the action or request of the original
registrant; some (the "preferred variant labels") are typically
registered automatically. The zone is expected to establish
appropriate policies for situations in which the variant forms of one
label conflict with already-reserved or already-registered labels.
Most of these concepts were introduced because of concerns about
specific issues with CJK characters, beginning from the requirement
that the use of Simplified Chinese by some registrants and
Traditional Chinese by others not be permitted to create confusion or
opportunities for fraud. While they may be applicable to registry
tables constructed for alphabetic scripts, the translation should be
done with care, since many analogies are not exact.
Some of the important issues are discussed in the sections that
follow, especially Section 3. The JET model may be considered as a
variation on, and inspiration for, the model and method presented by
the rest of this document, although the JET model has been completely
developed only for CJK characters. Other languages or scripts,
especially alphabetic ones, may require other variations.
1.4.2. Reserved Names and Label Packages
A basic assumption of the JET model is that, if the evolution of
specific characters or the properties of Unicode [Unicode]
[Unicode32] or IDNA cause two strings to appear similar enough to
cause confusion, then both should be registered by the same party or
one of them should become unregisterable. The definition of "appear
similar enough" will differ for different cultures and circumstance,
and hence DNS zones, but the principle is fairly general. In the JET
model, all of the variant strings are identified, some are registered
into the DNS automatically, and others are simply reserved and can be
registered, if at all, only by the original registrant. Other zones
might find other policies appropriate. For example, a zone might
conclude that having similar strings registered in the DNS was
undesirable. If so, the list of variant strings would be used only
to build a list of names that would be reserved and prohibited from
being registered.
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RFC 4290 IDN Registration Practices December 20051.5. Languages, Scripts, and Variants1.5.1. Languages versus Scripts
Conversations about scripts -- collections of characters associated
with particular languages -- are common when discussing character
sets and codes. However, the boundaries between one script and
another are not well-defined. The Unicode Standard ([Unicode],
[Unicode32]), for example, does not define script boundaries at all,
even though it is structured in terms of usually-related blocks of
characters. The issue is complicated by the common origin of most
alphabetic scripts in use in the world today (see, for example,
[Drucker] or the more scholarly [Daniels]).
Because of that history, certain characters (or, more precisely,
symbols representing characters) appear in the scripts associated
with multiple languages, sometimes with very different sounds or
meanings. This differs from the CJK situation in which, if a
character appears in more than one of the relevant languages, it will
usually have the same interpretation in each one. For the subset of
characters that actually are ideographs or pictographs, pronunciation
is expected to vary widely while meaning is preserved. At least in
part because of that similarity of meaning, it made sense in the JET
case to permit a registration to specify multiple languages, to
verify that the characters in the label string (the requested "Base
registration") were valid for each, and then to generate variant
labels using each language in turn. For many alphabetic languages,
it may be more sensible to prohibit the label string submitted for
registration from being associated with more than one language.
Indeed, "one label, one language" has been suggested as an important
barrier against common sources of "look-alike" confusion. For
example, the imposition of that rule in a zone would prevent the
insertion of a few Greek or Cyrillic characters with shapes identical
to the Latin ones into what was otherwise a Latin-based string. For
a particular table, the list of base characters may be thought of as
the script associated with the relevant language, with the
understanding that the table design does not prevent the same
character from appearing in the tables for multiple languages.
Indeed, this notion of a script that is local and specifically
identified can be turned around: so-called "language tables" are
associated with languages only insofar as thinking about the
character structure and word forms associated with a given language
helps to inform the construction of the table. A country like
Finland, for example, might select among:
o One table each for Finnish, Swedish, and English characters and
conventions, permitting a string to be registered in one, two, or
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all three languages. However, a three-language registration would
necessarily prohibit any characters that did not appear in all
three languages, since the label would make little sense
otherwise.
o One table each, but with a "one label, one language" rule for the
zone.
o A combined table based on the observation that all three writing
systems were based on Roman characters and that the possibilities
for confusion of interest to the registry would not be reduced by
"language" differentiation. This option raises an interesting
issue about language labeling as described in Section 1.4.1; see
the discussion in Section 7 below.
Regardless of what decisions were made about those languages and
scripts, they might have a separate table for registration of labels
containing Cyrillic characters. That table might contain some
Roman-derived characters (either as base characters or as variants),
just as some CJK tables do. See also Section 2, below.
Tables that present multiple languages, as described above, have
introduced confusion and discomfort among those who have failed to
understand these definitions. The consequence of these definitions
is that use of a language or script code in a registration is a
mnemonic, rather than a normative statement about the language or
script itself. When that confusion is likely to occur, it is
appropriate to simply use the registry identifier and a sequence
number to identify the registration.
As the JET Guidelines stress, no tables or systems of this type --
even if identified with a language as a means of defining or
describing the table -- can assure linguistic or even syntactic
correctness of labels with regard to that language. That assurance
may not be possible without human intervention or at least dictionary
lookups of complete proposed labels. It may even not be desirable to
attempt that level of correctness (see Section 2).
Of course, if any language-based tests or constraints, including "one
label, one language", are to be applied to limit the associated
sources of confusion, each zone must have a table for each language
in which it expects to accept registrations. The notion of a single
combined table for the zone is, in the general case, simply
unworkable. One could use a single table for the zone if the intent
were to impose only minimal restrictions, e.g., to force alphabetic
and numeric characters only, excluding symbols and punctuation. That
type of restriction might be useful in eliminating some problems,
such as those of unreadable labels, but it would be unlikely to be
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very helpful with, e.g., confusion caused by similar-looking
characters.
1.5.2. Variant Selection
The area of character variants is rife with difficulties (and perhaps
opportunities). There is no universal agreement about which base
characters have variants, or if they do, what those variants are.
For example, in some regions of the world and in some languages,
LATIN SMALL LETTER O WITH DIAERESIS (U+00F6) and LATIN SMALL LETTER O
WITH STROKE (U+00F8) are variants of each other, while in other
regions, most people would think that LATIN SMALL LETTER O WITH
STROKE has no variants. In some cases, the list of variants is
difficult to enumerate. For example, it required several years for
the Chinese language community to create variant tables for use with
IDNA, and it remains, at the time of this writing, questionable how
widely those tables will be accepted among users of Chinese from
areas of the world other than those represented by the groups that
created them.
Thus, the first thing a registry should ask is whether or not any of
the characters that they want to permit to be used have variants. If
not, the registry's work is much simpler. This is not to say that a
registry should ignore variants if they exist: adding variants after
a registry has started to take registrations will be nearly as
difficult administratively as removing characters from the list of
acceptable characters. That is, if a registry later decides that two
characters are variants of each other, and there are actively-used
names in the zones that differ only on the new variants, the registry
might have to transfer ownership of one of the names to a different
owner, using some process that is certain to be controversial.
This situation in likely to be much easier for areas and zones that
use characters that previously did not occur in the DNS at all than
it will be for zones in which non-English labels have been registered
in ASCII characters for some time, presumably because the language of
interest uses additional "Latin" characters with some conventions
when only ASCII is available. In the former case, the rules and
conventions can be established before any registrations occur. In
the latter, there may be conflicts or opportunities for confusion
between existing registrations and now-permitted Roman-based
characters that do not appear in ASCII. For example, a domain name
might exist today that uses the name of a city in Canada spelled as
"Montreal". If the zone in which it occurs changes its rules to
permit the use of the character LATIN SMALL LETTER E WITH ACUTE
(U+00E9), does the name of the city, spelled (correctly) using that
character, conflict with the existing domain name registration?
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Certainly, if both are permitted, and permitted to be registered by
separate parties, there are many opportunities for confusion.
Of course, zone managers should inform all current registrants when
the registration policy for the zone changes. This includes the
times when IDN characters are first allowed in the zone, when
additional characters are permitted, and when any change occurs in
the character variant tables.
Many languages contain two variants for a character, one of which is
strongly preferred. A registry might restrict the base registration
to the preferred form, or it might allow any form for the base
registration. If the variant tables are created carefully, the
resulting bundles will be the same, but some registries will give
special status to the base registration such as its appearance in
"Whois" databases.
1.6. Variants are not a Universal Remedy
It is worth stressing that there are many obvious opportunities for
confusion that variant systems, by virtue of being based on
processing of individual characters, cannot address. For example, if
a language can be written with more than one script, or
transliterations of the language into another script are common,
variant models are insufficient to prevent conflicting registration
of the related forms. Avoiding those types of problems would require
different mechanisms, perhaps based on phonetic or natural language
processing techniques for the entire proposed base registration.
1.7. Reservations and Exclusions1.7.1. Sequence Exclusions for Valid Characters
The JET Guidelines are based on processing only single characters.
Pairs or longer sequences of characters can, at the option of the
registry, be handled through what the Guidelines describe as
"additional processing". These registry-specific string processing
procedures are specifically permitted by the guidelines to supplement
the per-character processing that generates the variants.
A different zone with different needs could use a modified version of
the table structure, or different types of additional processing, to
prohibit particular sequences of characters by marking them as
invalid, and to accept characters by marking them as valid. Other
modifications or extensions might be designed to prevent certain
letters from appearing at the beginning or end of labels. The use of
regular expressions in the "valid characters" column might be one way
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to implement these types of restrictions, but there has been no
experience so far with that approach.
In particular, in some scripts derived from Roman characters,
sequences that have historically been typographically represented by
single "ligature" or "digraph" characters may also be represented by
the separate characters (e.g., "ae" for U+00E6 or "ij" for U+0133).
If it is desired to either prohibit these, or to treat them as
variants, some extensions to the single-character JET model may be
needed. Some careful thinking about IDNA (especially nameprep) may
also be needed, since some of these combinations are excluded there).
1.7.2. Character Pairing Issues
Some character pairings -- the use of a character form (glyph) in one
language and a different form with the same properties in a related
one -- closely approximate the issues with mapping between
Traditional and Simplified Chinese, although the history is
different. For example, it might be useful to have "o" with a stroke
(U+00F8) as a variant for "o" with diaeresis above it (U+00F6) (and
the equivalent upper-case pair) in a Swedish table, and vice versa in
a Norwegian one, or to prohibit one of these characters entirely in
each table. In a German table, U+00F8 would presumably be
prohibited, while U+00F6 might have "oe" as a variant. Obviously, if
the relevant language of registration is unknown, this type of
variant matching cannot be applied in any sensible way.
1.8. The Registration Bundle1.8.1. Definitions and Structure
As one of its critical innovations, the JET model defines an "IDN
package", known in this document as a "registration bundle", which
consists of the primary registered string (which is used as the name
of the bundle), the information about the language table(s) used, the
variant labels for that string, and indications of which of those
labels are registered in the relevant zone file ("activated" in the
JET terminology). Registration bundles are also atomic -- one can
not add or remove variant labels from one without unregistering the
entire package. A label exists in only one registration bundle at a
time; if a new label is registered that would generate a variant that
matches one that appears in an existing package, that variant simply
is not included in the second package. A subsequent de-registration
of the first package does not cause the variant to be added to the
second. While it might be possible to change this in other models,
the JET conclusion was that other options would be far too complex to
implement and operate and would cause many new types of name
conflicts.
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RFC 4290 IDN Registration Practices December 20051.8.2. Application of the Registration Bundle
A registry has three options for handling the case where the
registration bundle contains more than one label. The policy options
are:
o Register and resolve all labels in the zone, making the zone
information identical to that of the registered labels. This
option will allow end users to find names with variants more
easily, but will result in larger zone files. For some language
tables, the zone file could become so large that it could
negatively affect the ability of the registry to perform name
resolution. If the base registration contains several characters
that have equivalents, the owner could end up having to take care
of large numbers of zones. For instance, if DIGIT ONE is a
variant of LATIN SMALL LETTER L, the owner of the domain name all-
lollypops.example.com will have to manage 32 zones. If the intent
is to keep the contents of those zones identical, the owner may
then face a significant administrative problem. If other concerns
dictate short times to live and absolute consistency of DNS
responses, the challenges may be nearly impossible.
o Block all labels other than the registered label so they cannot be
registered in the future. This option does not increase the size
of the zone file and provides maximum safety against false
positives, but it may cause end users to not be able to find names
with variants that they would expect. If the base registration
contains characters that have equivalents, Internet users who do
not know what base characters were used in the registration will
not know what character to type in to get a DNS response. For
instance, if DIGIT ONE is a variant of LATIN SMALL LETTER L, and
LATIN SMALL LETTER L is a variant of DIGIT ONE, the user who sees
"pale.example.com" will not know whether to type a "1" or a "l"
after the "pa" in the first label.
o Resolve some labels and block some other labels. This option is
likely to cause the most confusion with users because including
some variants will cause a name to be found, but using other
variants will cause the name to be not found. For example, even
if people understood that DIGIT ONE and LATIN SMALL LETTER L were
variants, a typical DNS user wouldn't know which character to type
because they wouldn't know whether this pair were used to register
or block the labels. However, this option can be used to balance
the desires of the name owner (that every possible attempt to
enter their name will work) with the desires of the zone
administrator (to make the zone more manageable and possibly to be
compensated for greater amounts of work needed for a single
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registration). For many circumstances, it may be the most
attractive option.
In all cases, at least the registered label should appear in the
zone. It would be almost impossible to describe to name owners why
the name that they asked for is not in the zone, but some other name
that they now control is. By implication, if the requested label is
already registered, the entire registration request must be rejected.
2. Some Implications of This Approach
Historically, DNS labels were considered to be arbitrary identifier
strings, without any inherent meaning. Even in ASCII, there was no
requirement that labels form words. Labels that could not possibly
represent words in any Romance or Germanic language (the languages
that have been written in "Latin" scripts since medieval times or
earlier) have actually been quite common. In general, in those
languages, words contain at least one vowel and do not have embedded
numbers. As a result, a string such as "bc345df" cannot possibly be
a "word" in these languages. More generally, the more one moves
toward "language"-based registry restrictions, the less it is going
to be possible to construct labels out of fanciful strings. While
fanciful strings are terrible candidates for "words", they may make
very good identifiers. To take a trivial example using only ASCII
characters, "rtr32w", "rtr32x", and "rtr32z" might be very good DNS
labels for a particular zone and application. However, given the
embedded digits and lack of vowels, they, like the "bc345df" example
given above, would fail even the most superficial of tests for valid
English (or German or French (etc.)) word forms.
It is worth noting that several DNS experts have suggested that a
number of problems could be solved by prohibiting meaningful names in
labels, requiring instead that the labels be random or nonsense
strings. If methods similar to those discussed in this document were
used to force identifiers to be closer to meaningful words in real
languages, the result would be directly contradictory to those
"random name" approaches.
Interestingly, if one were trying to develop an "only words" system,
a rather different -- but very restrictive -- model could be
developed using lookups in a dictionary for the relevant language and
a listing of valid business names for the relevant area. If a string
did not appear in either, it would not be permitted to be registered.
Models that require a prior national business listing (or
registration) that is identical to the proposed domain name label
have historically been used to restrict registrations in some
country-code top level domains, so this is not a new idea. On the
other hand, if look-alike characters are a concern, even that type of
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rule (or restriction) would still not avoid the need to consider
character variants.
Consequently, registries applying the principles outlined in this
document should be careful not to apply more severe restrictions than
are reasonable and appropriate while, at the same time, being aware
of how difficult it usually is to add restrictions at a later time.
3. Possible Modifications of the JET Model
The JET model was designed for CJK characters. The discussion above
implies that some extensions to it may be needed to handle the
characteristics of various alphabetic scripts and the decisions that
might be made about them in different zones. Those extensions might
include facilities to process:
o Two-character (or more) sequences, such as ligatures and
typographic spelling conventions, as variants.
o Regular expressions or some other mechanism for dealing with
string positions of characters (e.g., characters that must, or
must not, appear at the beginning or end of strings).
o Delimiter breaks to permit multiple languages to be used,
separately, within the same label. E.g., is it possible to define
a label as consisting of two or more components, each in a
different language, with some particular delimiter to define the
boundaries of the components?
4. Conclusions and Recommendations About the General Approach
After examining the implications of the potential use of the full
range of characters permitted by IDNA in DNS labels, multiple groups,
including IESG [IESG-IDN] and ICANN [ICANN-IDN] [ICANN-IDN2], have
concluded that some restrictions are needed to prevent many forms of
user confusion about the actual structure of a name or the word,
phrase, or term that it appears to spell out. The best way to
approach such restrictions appears to draw from the language and
culture of the community of registrants and users in the relevant
zone: if particular characters are likely to be surprising or
unintelligible to both of those groups, it is probably wise to not
permit them to be used in registrations. Registration restrictions
can be carried much further than restricting permitted characters to
a selected Unicode subset. The idea of a reserved "bundle" of
related labels permits probably-confusing combinations or sets of
characters to be bound together, under the control of a single
registrant. While that registrant might still use the package in a
way that confused his or her own users (the approach outlined here
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will not prevent either ill-though-out ideas or stupidity), the
possibility of turning potential confusion into a hostile attack
would be considerably reduced.
At the same time, excessive restrictions may make DNS identifiers
less useful for their original purpose: identifying particular hosts
and similar resources on the network in an orderly way. Registries
creating rules and policies about what can be registered in
particular zones -- whether those are based on the JET Guidelines or
the suggestions in this document -- should balance the need for
restrictions against the need for flexibility in constructing
identifiers.
The discussion above provides many options that could be selected,
defined, and applied in different ways in different registries
(zones). Registrars and registrants would almost certainly prefer
systems in which they can predict, at least to a first order
approximation, the implications of a particular potential
registration. Predictability of that sort probably requires more
standards, and less flexibility, than the model itself might suggest.
5. A Model Table Format
The format of the table is meant to be machine-readable but not
human-readable. It is fairly trivial to convert the table into one
that can be read by people.
Each character in the table is given in the "U+" notation for Unicode
characters. The lines of the table are terminated with either a
carriage return character (ASCII 0x0D), a linefeed character (ASCII
0x0A), or a sequence of carriage return followed by linefeed (ASCII
0x0D 0x0A). The order of the lines in the table may or may not
matter, depending on how the table is constructed.
Comment lines in the table are preceded with a "#" character (ASCII
0x2C).
Each non-comment line in the table starts with the character that is
allowed in the registry and expected to be used in registrations,
which is also called the "base character". If the base character has
any variants, the base character is followed by a vertical bar
character ("|", ASCII 0x7C) and the variant string. If the base
character has more than one variant, the variants are separated by a
colon (":", ASCII 0x3A). Strings are given with a hyphen ("-", ASCII
0x2D) between each character. Comments beginning with a "#" (ASCII
0x2C), and may be preceded by spaces (" ", ASCII 0x20).
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The following is an example of how a table might look. The entries
in this table are purposely silly and should not be used by any
registry as the basis for choosing variants. For the example, assume
that the registry:
o allows the FOR ALL character (U+2200) with no variants
o allows the COMPLEMENT character (U+2201) which has a single
variant of LATIN CAPITAL LETTER C (U+0043)
o allows the PROPORTION character (U+2237) which has one variant
which is the string COLON (U+003A) COLON (U+003A)
o allows the PARTIAL DIFFERENTIAL character (U+2202) which has two
variants: LATIN SMALL LETTER D (U+0064) and GREEK SMALL LETTER
DELTA (U+03B4)
The table contents (after any required header information, see
[IANA-language-registry] and the discussion in Section 7 below) would
look like:
# An example of a table
U+2200
U+2201|U+0043
U+2237|U+003A-U+003A # Note that the variant is a string
U+2202|U+0064:U+03B4 # Two variants for the same character
Implementers of table processors should remember that there are tens
of thousands of characters whose codepoints are greater than 0xFFFF.
Thus, any program that assumes that each character in the table is
represented in exactly six octets ("U", "+", and four octets
representing the character value) will fail with tables that use
characters whose value is greater than 0xFFFF.
6. A Model Label Registration Procedure: "CreateBundle"
This procedure has three inputs:
1. the proposed base registration,
2. the language (or script, if the registration is script-based, but
"language" is used for convenience below) for the proposed base
registration, and
3. the processing table associated with that language.
The output of the process is either failure (the base registration
cannot be registered at all), or a registration bundle that contains
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one or more labels (always including the base registration). As
described earlier, the registration bundle should be stored with its
date of creation so that issues with overlapping elements between
bundles can later be resolved on a first-come, first-served basis.
There are two steps to processing the registration:
1. Check whether the proposed base registration exists in any
bundle. If it does, stop immediately with a failure.
2. Process the base registration with the mechanism described as
"CreateBundle" in Section 6.1, below.
Note that the process must be executed only once. The process must
not be performed on any output of the process, only on the proposed
base registration.
6.1. Description of the CreateBundle Mechanism
The CreateBundle mechanism determines whether a registration bundle
can be created and, if so, populates that bundle with valid labels.
During the processing, a "temporary bundle" contains partial labels,
that is, labels that are being built and are not complete labels.
The partial labels in the temporary bundle consist of strings.
The steps are:
1. Split the base registration into individual characters, called
"candidate characters". Compare every candidate character
against the base characters in the table. If any candidate
character does not exist in the set of base characters, the
system must stop and not register any names (that is, it must not
register either the base registration or any labels that would
have come from character variants).
2. Perform the steps in IDNA's ToASCII sequence for the base
registration. If ToASCII fails for the base registration, the
system must stop and not register any label (that is, it must not
register either the base registration or labels that might have
been created from variants of characters contained in it). If
ToASCII succeeds, place the base registration into the
registration bundle.
3. For every candidate character in the base registration, do the
following:
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o Create the set of characters that consists of the candidate
character and any variants.
o For each character in the set from the previous step,
duplicate the temporary bundle that resulted from the previous
candidate character, and add the new character to the end of
each partial label.
4. The temporary bundle now contains zero or more labels that
consist of Unicode characters. For every label in the temporary
bundle, do the following:
o Process the label with ToASCII to see if ToASCII succeeds. If
it does, add the label to the registration bundle. Otherwise,
do not process this label from the temporary bundle any
further; it will not go into the registration bundle.
The result of the processing outlined above is the registration
bundle with the base registration and possibly other labels.
6.2. The "no-variants" Case
It is clear that, for many scripts, registries will choose to create
tables without variants, either because variants are clearly not
necessary or because they are determined to cause more confusion and
overhead than is justified by the circumstances. For those
situations the table model of Section 5 becomes a trivial listing of
base characters and only the first two steps of CreateBundle
(verifying that all candidate character are in the base ("valid")
character list and verifying that the resulting characters will
succeed in the ToASCII operation) are applicable. Even the second of
those steps becomes pro forma if the advice in the next subsection is
followed.
6.3. CreateBundle and Nameprep Mapping
One of the functions of Nameprep, and IDNA more generally, is to map
a large number of Unicode characters (code points) into a smaller
number to avoid a different but overlapping set of confusion
problems. For example, when a non-ASCII script makes distinctions
between "upper case" and "lower case", nameprep maps the upper case
characters to the lower case ones in order to simulate the DNS
protocol's rule that ASCII characters are interpreted in a case-
insensitive way. Unicode also contains many code points that are
typographic variants on each other (e.g., forms with different widths
and code points that designate font variations for mathematical
uses), the Unicode standard explicitly identifies them that way, and
Nameprep maps these onto base characters.
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While having these mapping functions available during lookup may be
quite helpful to users who type equivalent forms, registrations are
probably best performed in terms of the IDNA base characters only,
i.e., those characters that nameprep will not change. This will have
two advantages.
o Registrants will never find themselves in the rather confusing
position of having submitted one string for registration and
finding a different string in the registry database (which could
otherwise occur even if the relevant language table does not
contain variants).
o Those who are interested in what characters are permitted by a
given registry will only need to examine the relevant tables,
rather than simulating the IDNA algorithm to determine the result
of processing particular characters.
7. IANA Considerations
Under ICANN (not IETF) direction and management, the IANA has created
a registry for language variant tables. The authoritative
documentation for that registry is in [IANA-language-registry].
Since the registry exists and is being managed under ICANN direction,
the material that follows is a review of the theory of this registry,
rather than new instructions for IANA.
As described above and suggested in the JET Guidelines, the
registration rules generally require only that:
o The application be submitted or endorsed by a TLD registry, to
ensure that someone cares about the particular table.
o The table be identified by the following:
* the name -- usually the top-level domain name -- of the
submitting or endorsing registry;
* one of: a language designation (consistent with [RFC3066] or
with some other system approved by the IANA), a script
designation, a combination of the two, or a sequence number
acceptable to IANA for this purpose;
* a version number; and
* a date.
o Characters listed in the table be identified by Unicode code
points, as discussed above.
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o The table format may correspond to that identified in [RFC3743],
or in Section 5 above, or may be some variation on those themes
appropriate to the local processing model (with or without
variants).
This raises some issues that will need to be worked out as
experiences accumulate. For example, more standardization of table
formats would be desirable to allow processing by the same computer
tools for different registries and languages. But standardization
seems premature at this time due to differences in languages,
processing, and requirements and lack of experience with them.
Similarly, if a registry concludes that it should use a table that
contains characters from several scripts, it is not clear how such a
table should be designated. Identifying it with a language code
(either according to [RFC3066] or an independent code registered with
IANA) is likely to just introduce more confusion, especially given
other Internet uses of the language codes. It appears that some
other convention will be needed for those cases, and it should be
developed (if it has not already been established by the time this
document is published).
8. Internationalization Considerations
This document specifies a model mechanism for registering
Internationalized Domain Names (IDNs) that can be used to reduce
confusion among similar-appearing names. The proposal is designed to
facilitate internationalization while permitting a balance between
internationalization concerns and concerns about keeping the Internet
global and domain name system references unique in the perception of
the user as well as in practice.
9. Security Considerations
Registration of labels in the DNS that contain essentially
unrestricted sequences of arbitrary Unicode characters may introduce
opportunities for either attacks or simple confusion. Some of these
risks, such as confusion about which character (of several that look
alike) is actually intended, may be associated with the presentation
form of DNS names. Others may be linked to databases associated with
the DNS, e.g., with the difficulty of finding an entry in a "Whois
file" when it is not clear how to enter or to search for the
characters that make up a name. This document discusses a family of
restrictions on the names that can be registered. Restrictions of
the type described can be imposed by a DNS zone ("registry"). The
document also describes some possible tools for implementing such
restrictions.
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While the increased number and types of characters made available by
Unicode considerably increases the scale of the potential problems,
the problems addressed by this document are not new. No plausible
set of restrictions will eliminate all problems and sources of
confusion: for example, it has often been pointed out that, even in
ASCII, the characters digit-one ("1") and lower case L ("l") can
easily be confused in some display fonts. But, to the degree to
which security may be aided by sensible risk reduction, these
techniques may be helpful.
10. Acknowledgements
Discussions in the process of developing the JET Guidelines were
vital in developing this document and all of the JET participants are
consequently acknowledged. Attempts to explain some of the issues
uncovered there to, and feedback from, Vint Cerf, Wendy Rickard, and
members of the ICANN IDN Committee were also helpful in the thinking
leading up to this document.
An effort by Paul Hoffman to create a generic specification for
registration restrictions of this type helped to inspire this
document, which takes a somewhat different, more language-oriented,
approach than his initial draft. While the initial version of that
draft indicated that multiple languages (or multiple language tables)
for a single zone were infeasible, more recent versions [Hoffman-reg]
shifted to inclusion of language-based approaches. The current
version of this document incorporates considerable text, and even
more ideas, from those drafts, with Paul Hoffman's generous
permission.
Feedback was provided by several registry operators (of both country
code and generic TLDs), including Edmon Chung and Ram Mohan of
Afilias, and by ICANN and IANA staff, notably Tina Dam and Theresa
Swinehart. This feedback about issues encountered in registering
tables and designing IDN implementations resulted in the addition of
significant clarifying text to the current version of the document.
The opinions expressed here are the sole responsibility of the
author. Some of those whose ideas and comments are reflected in this
document may disagree with the conclusions the author has drawn from
them. The first draft version of this document was posted in June
2003.
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